Refractometry:

Analyzing Results

Finding Refractive Indexes

One of the most common uses of the refractive index
is to compare the value you obtain with values listed in the literature.
This comparison is used to help confirm the identity of the compound and/or
assess its purity. The following sources list refractive indexes for a
wide variety of substances:

There are also many computer-based chemical databases
that contain refractive indexes. For example, both the CRC
Handbook of Chemistry and Physics, and The
Merck Index have computer-based versions. These can be particularly
useful if your sample is an unknown and you want to search for compounds
with similar indexes of refraction. One of the most comprehensive databases
for organic compounds is MDL's
Beilstein Crossfire database. (Last time I checked it contained 96
reported values for the index of refraction of isopropanol alone!) If
you don't have access to one of the commercial chemical databases, I recommend
The Organic Compounds
Database at Colby College which can be used on the web for no charge.

Comparing Refractive Indexes

Since the refractive index of a substance depends on
the wavelength it is important that the refractive index you are comparing
to was obtained at the same wavelength as the one you determined. This
is usually not an issue since the vast majority of refractive indexes
are obtained using the sodium D line at 589.3 nm. (Even refractometers
that use white light are normally constructed so that the refractive index
obtained corresponds to that for light at 589.3 nm.)

The refractive index also depends on the temperature.
Thus, it is best to obtain the refractive index of your sample at the
same temperature as the value you plan to compare with; in most cases
this will be 20 °C. However, if your refractometer is not equipped
with a temperature regulating system, you may simply be stuck with room
temperature, whatever that happens to be.

For most organic liquids the index of refraction decreases
by approximately 0.00045 ± 0.0001 for every 1 °C increase in
temperature. See Table 1 for a few examples. Note that the index of refraction
for water is much less dependent on temperature than most organic liquids,
decreasing by about 0.0001 for every 1 °C increase in temperature.

If you determined your index of refraction at a different
temperature than that reported in the literature you will need to correct
your value for the temperature variation before comparing it to the literature
value. For example, if you determined the index of refraction of an organic
liquid at 24°C, and want to compare it to a literature value determined
at 20 °C, you should subtract 4(0.00045) = 0.0018 from the index of
refraction you obtained.

Figure 1. Equation for estimating
the index of refraction at a temperature different than that used
for the measurement. This method relies on the observation that the
temperature variation in the index of refraction is similar for many
organic liquids. This correction is only approximate
and should not be used for aqueous solutions.

A typical laboratory refractometer can determine the
refractive index of a sample to a precision of ± 0.0002. However,
small amounts of impurities can cause significant changes in the refractive
index of a substance. Thus, unless you have rigorously purified
your compound, a good rule of thumb is that anything within ± 0.002
of the literature value is a satisfactory match.

Another possible source of error is miscalibration of
the refractometer. This is readily checked by using a sample of known
refractive index. Distilled water is a particularly convenient standard
since it is nontoxic, readily available in pure form, and its refractive
index varies only slightly with temperature (Table 1). If you find that
the index of refraction of the standard is consistently off by more than
0.0005 from the expected value report this to your instructor or the person
in charge of calibrating the refractometer.

Probably the most common source of error in analog refractometers
is misreading of the scale. If the index of refraction you determined
seems inconsistent with other data, try repeating the measurement.

Determining Concentrations of Solutions

Determining the concentration of a solute in a solution
is probably the most popular use of refractometry. For example, refractometer-based
methods have been developed for determining the percentage of sugar in
fruits, juices, and syrups, the percentage of alcohol in beer or wine,
the salinity of water, and the concentration of antifreeze in radiator
fluid. Many industries use refractometer-based methods in quality control
applications.

In most cases the refractive index is linearly (or nearly
linearly) related to the percentage of dissolved solids in a solution
(Figure 2). By comparing the value of the refractive index of a solution
to that of a standard curve the concentration of solute can be determined
with good accuracy. Many refractometers contain a "Brix" scale
that is calibrated to give the percentage (w/w) of sucrose dissolved in
water.

Structural Information

The refractive index does not provide detailed information
about a molecule's structure, and it is not usually used for this purpose
since spectroscopic techniques are much more powerful at revealing details
of molecular structure. One structural factor that influences the refractive
index of a sample is its polarizability. Substances containing more polarizable
("soft") groups (e.g., iodine atoms or aromatic rings) will
normally have higher refractive indexes than substances containing less
polarizable ("hard") groups (e.g., oxygen atoms or alkyl groups).
See Table 2 below.